The present invention relates to supported transition metal transesterification catalysts. It also relates to a method for preparation of such catalysts.
Simple and polymeric esters are major products of the chemical industry. As such, there are a wide variety of processes for their production. These include direct esterification by reactions of alcohols with carboxylic acids or anhydrides as well as various interchange reactions including alcoholysis, in which the alcohol moiety of an ester is exchanged by another alcohol, acidolysis, in which the carboxylic acid moiety is exchanged by another carboxylic acid, and transesterification in which the alcohol moieties of two different esters exchange with each other. These interchange reactions will be collectively referred to as transesterification reactions.
In the absence of some type of catalyst, esterification and transesterification reactions tend to be quite slow. For this reason reactions are almost always catalyzed. Acids, bases, and transition metal based catalysts are all used in various applications. However, there are a number of problems associated with the use of acidic and basic catalysts. These catalysts often promote undesirable side reactions which can make it difficult to isolate a pure product without employing extensive purification procedures. Furthermore, they also often require neutralization at the end of the reaction as well as removal from the product. This again may entail extensive purification procedures.
To avoid many of the problems associated with acidic or basic catalysts, transition metal catalysts are gaining increased use. In addition to avoiding problems with neutralization associated with the acidic and basic catalysts, they are often more selective in their activity, thus avoiding unwanted side reactions. Transition metal catalysts can be roughly divided into two classes, homogeneous and heterogeneous. Homogeneous catalysts are soluble in the reaction medium. Because of this they suffer from one of the major problems of many acidic and basic catalysts. That is, removal of the catalyst at the end of the reaction is difficult, if not impossible. Often even trace amounts of transition metal impurities in products are intolerable and, therefore, complex steps are often needed to reduce transition metal content to acceptable levels. This results in additional processing steps, waste, and/or yield losses. Furthermore, homogeneous catalysts are often destroyed during removal. This "once through" utilization of the catalyst can result in unacceptably high manufacturing costs.
Heterogeneous catalysts are relatively insoluble in the reaction medium. As a result, they avoid many of the purification problems associated with acidic, basic, and homogeneous catalysts. Often they can be removed from the product by a simple filtration step. However, since their activity occurs at the catalyst surface, rather than in solution, heterogeneous catalysts tend to have low activity.
Thus, the goal of much catalyst research is to discover heterogeneous catalysts which are not only selective and easily removed from the reaction mixture, but are also highly active.
U.S. Pat. No. 4,043,941 (hereinafter '941) and U.S. Pat. No. 4,032,550 (hereinafter '550) describe the preparation of heterogeneous transesterification catalysts with high activity and good stability which are free-flowing powders. Preparation is accomplished by heating a solid hydroxylic support with a molar excess (based on support surface hydroxyl groups) of a transition metal alkoxide in an aliphatic hydrocarbon solvent in the presence of water. The transition metal alkoxide reacts with the surface hydroxyl groups and then is further hydrolzed by water resulting in a highly crosslinked matrix of transition metal atoms with bridging oxygen linkages. For activity, an excess of the transition metal over surface hydroxyl groups is required. This excess can be from 1:1 up to about 10.sup.6 :1. Water is required for preparation of the catalyst; at least one mole for each bridging oxygen atom.
Although the procedure used in the '941 and '550 patents produces catalysts with what would be considered generally high activity, a need still exists for heterogeneous transesterification catalysts with even higher activity. Thus, an objective of this invention was to prepare heterogeneous transesterification catalysts with higher activities than currently available. A further objective was to prepare these catalysts using anhydrous support materials in the absence of water. A still further objective was to prepare esters via transesterification using the improved catalysts.
We have found that transesterification catalysts with unexpectedly high activity can be prepared by a unique modification of the catalyst preparation procedure used in '941 and '550. Rather than the one step process of '941 and '550, our invention requires two steps. In the first, a transition metal alkoxide is partially hydrolyzed to form an oligomer. This oligomer is then reacted with a hydroxylic support in the second step to form the heterogeneous catalyst. The advantages of the two step process will become apparent from the following disclosure.